In general, a synchronous motor refers to a constant speed motor (a kind of an alternating current motor) that rotates at a constant frequency at a constant speed (synchronous speed) regardless of a load. This synchronous motor is typically configured with 3-phase windings arranged to form a rotating magnetic field on a stator and magnets (permanent magnets or electromagnets) embedded to form magnetic poles (N and S poles) in a rotor having a laminated structure of core members (iron cores).
The synchronous motor as described above is generally driven by the rotating magnetic field generated between the stator and the rotor. The synchronous motor having this driving method typically has a characteristic of rotating at a speed that is proportional to a frequency of supply voltage applied to the 3-phase windings and is inversely proportional to the number of magnetic poles of the rotor, i.e., at a speed that is synchronized with a frequency of input voltage.
However, since electromagnetic force acts on only one corresponding face, a conventional motor or generator requires the iron core that is a passage of strong electromagnetic force, and is reduced in efficiency due to a loss of the lines of electromagnetic force applied to the iron core.